Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers or substrates formed of electrically conductive sheet material. The electrode substrate has a porous structure that renders it permeable to fluid reactants and products in the fuel cell. The MEA also includes an electrocatalyst, typically disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reaction in the fuel cell. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load. At the anode, the fluid fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst. At the cathode, the fluid oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (so-called "direct methanol" fuel cells) and an oxygen-containing oxidant stream, such as air (or substantially pure oxygen) supplied to the cathode, the methanol is oxidized at the anode to produce protons and carbon dioxide. Typically, the methanol is supplied to the anode as an aqueous solution or as a vapor. The protons migrate through the ion exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations: EQU Anode reaction: CH.sub.3 OH+H.sub.2 O.fwdarw.6H.sup.+ +CO.sub.2 +6e.sup.- EQU Cathode reaction: 3/2O.sub.2 +6H.sup.+ +6e.sup.- .fwdarw.3H.sub.2 O
In electrochemical fuel cells, the MEA is typically interposed between two separator plates or fluid flow field plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. Fluid flow field plates typically have channels, grooves or passageways formed therein to provide means for access of the fuel and oxidant streams to the porous anode and cathode layers, respectively.
The porous electrode substrate material is electrically conductive to provide a conductive path between the electrocatalyst reactive sites and the current collectors. Materials commonly used as electrode substrate materials in solid polymer electrochemical fuel cells include:
(a) carbon fiber paper; PA1 (b) woven carbon fabric--optionally filled with carbon particles and a binder; PA1 (c) metal mesh or gauze--optionally filled with carbon particles and a binder. PA1 (a) filling at least two preformed webs, the webs of low electrical conductivity, with an electrically conductive filler; PA1 (b) laminating the at least two filled webs together.
Thus typical electrode substrate materials are preformed, highly electrically conductive, macroporous sheet materials which may contain a particulate electrically conductive material and a binder.
For electrode substrates comprising a macroporous sheet material (hereinafter referred to as a "web") filled with electrically conductive materials, the web need not be highly electrically conductive and in fact may be an electrical insulator. Electrode substrates, which are made from filled, poorly electrically conductive webs, have performance characteristics in fuel cells approaching those of conventional substrates. However, the use of preformed webs which are made from poorly conducting or insulating materials in the electrode substrate can offer offsetting advantages such as reduced cost, improved chemical compatibility and resistance to degradation and corrosion in fuel cell operation, improved mechanical properties including strength, durability and dimensional stability, and improved manufacturability.